Most conventional carpets comprise a primary backing with yarn tufts in the form of cut or uncut loops extending upwardly from this backing to form a pile surface. For tufted carpets, the yarn is inserted into a primary backing (frequently a woven or nonwoven material) by tufting needles and a pre-coat (i.e., a binder) is applied thereto.
Most residential and commercial carpets are also manufactured with a woven scrim (typically made from polypropylene) attached to the back of the carpet to provide dimensional stability to the carpet. The scrim is attached to the pre-coated carpet back with another binder formulation typically referred to as a skip coat or adhesive coating. The skip coat is applied to the scrim, and the scrim is then applied to the pre-coated backing of the carpet before the assembled carpet elements are sent into a curing oven. The purpose of the skip coat or adhesive coating is to provide a layer of material which will adhere the woven scrim to the back of the carpet.
For both the pre-coat and the skip coat, the physical properties of the binder are important to their successful utilization as carpet coatings. In this regard, there are a number of important requirements which must be met by such coatings. The coating must be capable of being applied to the carpet and dried using the processes and equipment conventionally employed in the carpet industry for latex, e.g. emulsion, coating. The binder composition must provide excellent adhesion to the pile fibers to secure them firmly in the backing. The coating will also typically have a high loading of fillers such as calcium carbonate, clay, aluminum trihydrate, barite, feldspar, cullet, flyash and/or recycled carpet backing.
The binders in coating compositions for carpet materials are frequently emulsion polymers, i.e., latex compositions, which can comprise copolymers of vinyl esters (such as vinyl acetate) and ethylene. Carpet coating compositions based on vinyl ester/ethylene copolymers are disclosed, for example, in U.S. Pat. Nos. 4,735,986; 5,026,765; 5,849,389 and 6,359,076 and in U.S. Patent Application Publication No. 2005/0287336. These copolymers are prepared by polymerizing appropriate co-monomers in an aqueous emulsion. Such emulsions can be stabilized by adding conventional surfactants (anionic, nonionic, cationic) as emuslifiers. Such emulsions may also be stabilized by including protective colloids such as those based on polyvinyl alcohols (PVOH), ionically modified starches, water-soluble starches, starch ethers, polyacrylic acid, carboxymethyl cellulose, natural gums, gelatin, synthetic polymers, or water-soluble cellulose ethers such as hydroxyethyl cellulose (HEC).
Substantially all-surfactant-based vinyl ester/ethylene (VAE) latex emulsions (i.e., those containing very little or no protective colloid as emulsion stabilizers) are especially desirable from the standpoint of permitting effective compounding of the emulsion with the various types of filler materials which are used in carpet coating compositions. Substantially all-surfactant stabilized binder emulsions also provide excellent compatibility with other materials typically used by the carpet industry in carpet manufacture such as styrene-butadiene rubber (SBR) emulsions.
Notwithstanding such filler compounding and compatibilty benefits, substantially all-surfactant-based vinyl ester/ethylene emulsions, when serving as carpet coating compositions, can lead to some processing problems during carpet manufacture. In particular, such processing problems can manifest themselves when the carpet containing the all-surfactant binder emulsion exits the curing oven at 110° C.-120° C. and then travels over a series of guide rollers and possibly through a shearing machine. Substantially all-surfactant stabilized coating compositions have a tendency to transfer to the rollers and create build-up which can cause maintenance issues. In addition, at times this build-up can transfer back to the carpet which can cause gouges in the face of the carpet as it goes through the shearing machine.
Attempts have been made to retain the benefits of substantially all-surfactant-based latex binders while minimizing the processing problems hereibefore described. Such attempts, for example, have involved the use of vinyl ester-ethylene based emulsions stabilized with both surfactant emulsifiers and protective colloids such as PVOH. While some of such attempts have been somewhat successful, these mixed emulsifier/colloid systems must be very precisely formulated and even then still have a tendency not to provide all of the benefits which could be realized by the use of either type of stabilizer system alone. Accordingly, there continues to be a need to identify carpet coating compositions based on vinyl ester/ethylene copolymer emulsions which exhibit all the benefits of substantially all-surfactant stabilized emulsions but which do not result in an unacceptable incidence of adverse processing problems during carpet manufacture.